Biomarker indicators for anaerobic oxidizers of methane in brackish-marine sediments with diffusive methane fluxes

The anaerobic oxidation of methane (AOM) is a major methane sink in marine sediments and plays a crucial role in mitigating methane fluxes to the overlying water column. We investigated biomarker distributions and compound specific isotopic signatures in sediments from sites on the Northern European continental margin that are characterized by a diffusive flux of methane. At all sites, the organic matter (OM) is predominantly derived from terrestrial higher plants, with subordinate abundances of algal biomarkers, but biomarkers for archaea and bacteria are also present. The co-occurrence of the archaeal lipids archaeol, sn-2-hydroxyarchaeol and 2,6,10,15,19-pentamethylicosane (PMI) with non-isoprenoidal glycerol diethers inferred to derive from sulfate-reducing bacteria (SRB) is similar to microbial biomarker assemblages observed at cold seeps. The archaeal and inferred SRB biomarker concentrations typically reach maxima close to the sulfate-methane transition zone (SMTZ), where archaeal biomarkers are depleted in ¹³C. The 16S rRNA gene sequences from the SMTZ of the Aarhus Bay sediment core indicate the occurrence of ANME-1 archaea, consistent with inferences derived from biomarker distributions. The observations suggest that AOM in these diffusive settings is mediated by consortia of archaea and bacteria similar to those found at many seep and methane hydrate sites around the world.